DM1 is caused by a CTG trinucleotide repeat expansion in the dystrophia myotonica protein kinase (DMPK) gene, located on chromosome 19q13.3, whereas DM2 is caused by a CCTG tetranucleotide repeat expansion in intron 1 of the nucleic acid-binding protein gene (CNBP), also known as zinc finger 9 gene (ZNF9), located on chromosome 3q21. Here, DMPK is linked to myotonic dystrophy type 1.